1. Field of the Invention
The present invention relates to a rotary disk filter element for a liquid filter, especially a beverage filter for biological filtration, with depth effect.
2. Description of the Prior Art
With one known filter element of this general the (German Offenlegungsschrift No. 24 52 524--Ziller dated May 13, 1976 and belonging to the assignee of the present invention), only the radially inner and outer portions of a filter fabric are secured to the filter element by being clamped between collar regions of adjacent filter elements and a radially outer clamping or securing ring placed upon the outer edge of the filter element.
Rotary disk filter elements of this type are used as a filter unit in liquid filters for the filtration of beverages. When filtration has been terminated after a number of hours or even days, filter residue retained by filter fabrics is centrifuged-off the latter by rotating the filter elements. In order to flush the remainder of the impurities still sticking to the filter fabrics, water, along with air, is flushed as wash fluid through the filter fabric counter to the direction of filtration, with the filter element subsequently being rotated in order to remove the residues or impurities that are still present in the filter element. In so doing, the water/air mixture still present in a support fabric is pressed at increased velocity through the filter fabric in order to completely flush out the residue still sticking to the filter fabric. With this extremely effective flushing/cleaning process, the filter fabric is generally subjected to very great mechanical tensile and shearing stresses since the filter fabric is not supported against the base plate counter to the direction of filtration. Since the air added for the cleaning process impedes the discharge of the wash fluid, the filter fabric is raised between the clamping locations counter to the direction of filtration. When, after the filter elements have been flushed, the supply of water and air is terminated, the filter fabric remains raised since the air settles in the fabric passages, as a result of which the pressure below the filter fabric is maintained. During the subsequent acceleration of the filter elementsvia rotation, the water/air mixture that has accumulated below the filter fabric is pressed at increased velocity through the filter fabric, thus unduly stressing the filter fabric beyond the permissible stretching limit. As a consequence of centrifugal force, the water/air mixture is pressed into the radially outer edge region of the filter element. At this location the water/air mixture leaves the filter element, and the impurities are flushed out.
During the subsequent filtration, the over-stretched filter fabric lies upon the base plate or support fabric accompanied by the formation of wrinkles and folds, since due to its great width the filter fabric no longer has enough space on the base plate. These folds adversely affect the filtering process, since at some locations the surface of the filter fabric is inhomogeneous and uneven. During the subsequent flushing processes, the folds are drawn open due to the lifting force acting thereon, whereupon great bending stresses act upon the fabric in the folds. As a result of the constantly changing bending stresses that act upon the folds during flushing, fatigue failures occur, especially in the region of the corners of the folds. At that point the filter element becomes unusable, so that it must be replaced. This is very expensive, especially with large filter elements due to their high manufacturing and assembly costs. The cost is magnified even further because in order to replace filter elements the entire filter unit has to be disassembled, and annular seals disposed between adjacent filter elements have to be removed and replaced, for which purpose also the filter unit has to be disassembled.
The filter fabric can also be stressed and damaged during flushing by the fact that over a long period of time the filter fabric is not sufficiently cleaned, for example by an inadequate water flushing. As a result, residue of filtering aids accumulates in the passages of the filter fabric, so that these passages become narrower. During flushing, the entire pressure of the flushing water then builds up below the unsupported fabric, so that the latter is mechanically overloaded, which again leads to a permanent overstretching of the fabric and, due to the alternating stressing of the filter fabric during filtration and flushing, also leads to the formation of folds and the aforementioned drawbacks connected therewith.
Overstretching of the fabric can also occur if too great a quantity of water per unit area is used for flushing, or if the water pressure or mechanical dynamic stresses are too great during flushing.
Chemical filters are also known where the filter fabric or intermediate layer of a support fabric is secured by rivets directly and in a non-planar manner on a perforated plate. However, these chemical filters are not cleaned by flushing with a wash fluid; rather, to clean the filter element in the filtration device the filter element is sprayed with water at very high pressure. For this purpose, the water is sprayed at an angle onto the surface of the filter element, so that the residues are taken along and the water flows off from the same side of the filter element or filter fabric. In contrast to the aforementioned state of the art, the filter fabric cannot be raised from the perforated plate during cleaning, since it is actually pressed against the perforated plate by the water stream, and is supported by the plate. It is therefore not necessary to secure the filter fabric, because the high water stream pressure is effected at an angle to the fabric, whereby locally great shearing forces occur.
An object of the present invention is to design a rotary disk filter element of the aforementioned general type in such a way that formation of folds due to excessive stretching and bulging of the filter fabric counter to the direction of filtration during flushing is reliably prevented.